Many of them are still slogging uphill to recoup share price losses incurred from the Brexit vote in the U.K. European investment banking revenue overall is down 23 percent this year compared with the same period in 2015, according to data tracker Dealogic. And all are lagging behind U.S. banks for wallet share, or how much revenue they take in from dealmaking compared to competitors.

JPMorgan Chase tops every bank in the EU for wallet share, with 7.3 percent of deals, according to data from Dealogic this week. It's followed by Goldman Sachs, which has 6.2 percent of deals, and only then, in third place, is an EU bank: Deutsche Bank has 5 percent of revenue on European mergers and acquisitions. But European banks (and their American counterparts) are fighting off a rising tide of boutique banks that have taken a growing percentage of M&A revenue from them over the last decade.

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Around the world, M&A levels have declined from recent record highs. But the pain is exacerbated in Europe, where big banks experienced a steeper drop off in revenue. Dealogic data show that investment banking in Germany, for example, is down 45 percent. Globally, European deals account for just 22 percent of banking revenue, the lowest margin since Dealogic began tracking investment banking wallet share.